This invention relates to radio frequency (microwave) systems, and, more particularly, to a reflective diode linearizer used in such systems.
Amplifiers are provided in space communications systems to amplify a radio frequency signal prior to transmission. The amplifiers, such as traveling wave tube amplifiers, klystrons, or solid-state power amplifiers, are often required to simultaneously amplify several different carriers. As a result, nonlinear intermodulation products are generated which may interfere with the processing of the transmitted signals.
The intermodulation products may be reduced by reducing the power output of the amplifier. This backoff of the output power operating point reduces signal levels and transmission efficiency, and may require the use of larger, heavier amplifiers.
In another approach, the signals may be processed to alter them to account for the intermodulation effects. Negative feedback, feed-forward, and predistortion techniques are known. In the latter, the signal provided to the amplifier is altered by generating a transfer characteristic which is the opposite of the saturation characteristic of the amplifier in both magnitude and phase, and this altered signal is provided to the amplifier. The result is that the amplified signal does not have the intermodulation products.
Reflective diode linearizers are known for use in predistortion techniques. However, the inventor has observed that these known reflective diode linearizers are based on unbalanced vector summation approaches. The result is poor input and output VSWR (voltage standing wave ratio).
There is therefore a need for an improved approach to the design of reflective diode linearizers and to the predistortion techniques used in space communications systems and other radio frequency communications systems. The present invention fulfills this need, and further provides related advantages.